1. Field of the Invention
The present invention relates to glass and chemically tempered glass to be used for operation panels of AV equipment, OA equipment, etc., doors of such products, operation buttons and knobs, accessories, etc.
2. Discussion of Background
Heretofore, for operation panels of AV equipment, OA equipment, etc., metallic panels of e.g. aluminum, or black panels have been heavily used. Particularly for AV equipment, etc., black panels have been heavily used, which are not highly noticeable so as not to disturb concentration on images or music when used, and which add a touch of class and expensive-looking.
For such black panels and operation buttons, knobs and the like thereof, usually a black resin or a metal component coated black has been used. However, with respect to a surface treated product e.g. by coating, the coated surface is likely to be damaged, and the coating may be removed with time. Further, with respect to a black resin which is a colored substrate itself, it is free from worry about removal, but the resin itself is likely to be damaged and in addition, its texture is cheap, and accordingly it is improper to a classy and elegant image required for high-grade products in some cases.
Further, the same applied to doors which conceal operation buttons of the above equipment. A metal should be colored in some way, and a resin is insufficient in rigidity and flatness and distortion of its surface will impair a touch of class and expensive-looking.
Accordingly, for some products, glass has been used as substrates of operation panels or doors. In a case where the substrate is made of glass, differently from a resin, it is hardly damaged, has sufficient rigidity, and provides a highly flat surface by polishing. However, commonly used glass is transparent, and accordingly it is used as a black panel by coating the rear side of a glass plate with an organic resin coating material or by coating it with a low-melting glass containing a black pigment, followed by firing. Glass the rear side of which is coated black provides surface reflection and rear-side reflection as observed from the non-coated side, and provides twinkling impressions.
On the other hand, in order not to cause such rear-side reflection, black glass having a colorant added to glass itself is used in some cases. Black glass, which does not provide the rear-side reflection, is likely to produce jet-black heavy impressions.
In a case where the opening of a metal casing or the like is covered with glass as a structural material or in a case where black glass is used for a door, it is required to increase the strength of the glass, although it is not necessary in a case where black glass is bonded to the front of the metal casing or the like for the purpose of decorating the operation panel.
As a method of increasing the strength of the glass, a means of forming a compressive stress layer on the glass surface is common.
As the means of forming a compressive stress layer on the glass surface, an air-cooling tempering method (thermal tempering method) of quenching the surface of a glass plate heated to the vicinity of its softening point by air-cooling or the like, and a chemical tempering method of replacing alkali metal ions (typically Li ions, Na ions) having a small ion radius on the surface of a glass plate with alkali ions (typically K ions) having a larger ion radius by ion exchange at a temperature of at most the glass transition point, are typically mentioned.
The above-described decorative glass is used usually in a thickness of at most 2 mm in many cases. If the air-cooling tempering method is applied to such a thin glass plate, it will be difficult to form a compressive stress layer since there is hardly a difference in the temperature between the surface and the interior, whereby the desired property of high strength will hardly be obtained. Further, by the air-cooling tempering, the flatness is likely to be impaired particularly with respect to a thin plate due to dispersion of the cooling temperature, whereby the texture which is the object of the present invention may be impaired. From such a viewpoint, the glass is preferably tempered by the latter chemical tempering method.
As glass which may be chemically tempered and which is black, glass disclosed in Patent Document 1 has been known. The glass disclosed in Patent Document 1 comprises aluminosilicate glass and a high concentration iron oxide incorporated therein.    Patent Document 1: JP-B-45-16112
In Examples disclosed in the above Patent Document 1, arsenious acid is used as a clarifying agent. Arsenious acid is a poisonous substance, and has harmful effect over the environment not only in the production step but also through the life cycle of a product is considered.
Accordingly, the present inventors have prepared and melted glass starting materials so as to achieve the glass composition as disclosed in Examples of Patent Document 1 without addition of arsenic acid, whereupon it was found that bubble removal i.e. defoaming properties were very poor, and only glass having a large number of remaining bubbles was obtained. That is, the molten glass was cast into a form of a block, which was sliced into a sheet, and its surface was polished, whereupon a large number of dents formed by cutting the bubbles in the glass (hereinafter referred to as open bubbles) were exposed on the polished surface.
For the above-described decorative application, since glass having such open bubbles cannot be used from the requirement in terms of the quality of the appearance, the product yield is extremely decreased. Further, the open bubbles may cause fracture, thus decreasing the strength.